Nanoharp hits high noteEach of the strings is only 150
atoms wideBy BBC News Online Science Editor
Dr David Whitehouse

First they brought you the world's
smallest guitar, now they bring you world's smallest musical instrument
- the nanoharp.

The microscopic guitar made by Cornell
University researchers two years ago was just a whimsical demonstration
of new nanofabrication technology.

But the nanoharp is a true stringed
instrument that plays real music, if only we could hear it. It plays a
tune too high for even a dog to hear.

The strings viewed from
the side

The nanoharp's strings vibrate at frequencies
as high as 380 MHz and are probably the fastest human-made moving object.
The human ear cannot hear frequencies above 22 kHz.

"This is another use for our new
ability to make microscopic mechanical systems," said Harold Craighead,
Cornell's professor of applied and engineering physics, who supervised
the research.

"By making things very small you
bring out properties that aren't evident in larger materials. We can combine
this information
with other types of measurements made by researchers in materials science
to help understand how materials behave."

The new device is carved out of a
single crystal of silicon.
The strings are actually silicon rods 50 nanometres (nm) in diameter, ranging
from about 1,000 to 8,000nm long. A nanometre is one billionth of a meter,
making each string about 150 atoms thick.

The entire device is about the size
of a red blood cell.

Cornell University scientists hope
to use it to study the properties of very small vibrating systems - ones
that some day could be used to make extremely sensitive chemical sensors,
for example.

Vibrating strings

The researchers are studying resonance
effects
in these microscopic systems.

The nanoguitar

They make the silicon rods vibrate by
applying a radio frequency voltage signal through the silicon base.

Then they measure the resulting vibrations
by bouncing laser light
off
the strings and observing the reflected light with a sensitive interferometer.

The researchers have measured the
highest frequency man-made vibrating strings, and the smallest vibrating
strings, smaller by a factor of four than anyone else has measured. Many
aspects of the nanoharp's behaviour have yet to be explained.

As with a full-sized harp, the resonant
frequency at which one of these tiny strings vibrates depends on the length
and the mass.

However, scientists say that the
microscopic strings are not under tension like those in a musical instrument,
and the resonant frequency of the nanoharp's strings follows a different
rule, varying as the square of the length, like a metal bar struck by a
hammer.